Cobalt Polypyridyl Complexes as Redox Mediators for Lipoamide Dehydrogenase

A series of cobalt polypyridyl complexes were synthesized and characterized by cyclic voltammetry in aqueous and acetonitrile solutions. Their mediation ability toward FADH2 oxidation in a monoenzyme (lipoamide dehydrogenase) and a bienzyme (lipoamide dehydrogenase coupled to lactate dehydrogenase) system was studied. Of all the complexes tested, only the [Co(tpy)2](PF6)2, [Co(NH2-phen)3](PF6)2 and [Co(CH3-phen)3](PF6)2 showed a current enhancement indicative of an ECcat mechanism in the presence of lipoamide dehydrogenase and NADH. No cross reactivity was observed for these complexes in the presence of lactate dehydrogenase/ NAD+ and lactate. The results also suggest that factors such as hydrophobic interactions, steric hindrance, H-bonding and driving force are important when designing a redox mediator for a biosensor. Protein and/or cofactor adsorption fouled the electrode surface lowering the current response. A self-contained bienzymatic biosensor exhibited a linear response (peak current versus log [lactate]) for lactate concentrations between 5 μM to 5 mM with a lowest detectable lactate concentration of 4.76 μM.